Silicon nitride ceramic and cutting tool made thereof

ABSTRACT

Provided is a silicon nitride based ceramic which is particularly useful for use as a cutting tool in the high speed chip forming machining of metallic materials. The ceramic is preferably composed of at least 85 volume percent (v/o) beta silicon nitride phase and less than about 5 v/o intergranular phase. The ceramic has greater than 0.2 weight percent (w/o) magnesia, greater than 0.2 w/o yttria, where the sum of magnesia and yttria is less than 5 w/o. The ceramic preferably has less than 0.2 v/o porosity.

This is a continuation of application Ser. No. 08/004,022 filed on Jan.15, 1993, now U.S. Pat. No. 5,382,273.

BACKGROUND OF THE INVENTION

The present invention relates to silicon nitride based ceramics andtheir use, particularly as cutting tools.

In the past, it has been taught by U.S. Pat. No. 4,652,276 that betasilicon nitride compositions useful to machine cast iron must containboth yttrium oxide (yttria) and magnesium oxide (magnesia) in the rangeof 5 to 20 weight percent (w/o), total, to obtain long tool life (i.e.improved wear resistance) and improved chipping resistance in themachining of nodular cast iron.

Y₂ O₃ and MgO are added in the amounts indicated to produce a glassyintergranular phase during sintering in an amount necessary to theachievement of the proper densification of the ceramic and improvedmetal cutting performance.

It was found that compositions composed of 98 w/o Si₃ N₄ -1 w/o MgO-1w/o Y₂ O₃ have poor chipping resistance and poor wear resistancecompared to the compositions in accordance with U.S. Pat. No. 4,652,276(see col. 4, tables I and II).

There, however, remains a need for more advanced silicon nitrideceramics and cutting tools made therefrom which have improved propertiesand cutting performance, but can also be densified by economicaldensification methods.

BRIEF SUMMARY OF THE INVENTION

Applicants have now discovered an improved silicon nitride based ceramiccomposition having improved metal cutting performance, mechanical andphysical properties over the prior art.

Their discovery is surprising in that their silicon nitride basedceramic composition contains less than 5 w/o total of yttrium oxide andmagnesium oxide, which is contrary to the teaching of the prior art. Inaddition, despite using a composition which is contrary to the priorart, the present invention preferably and unexpectedly has improvedhardness at elevated temperatures such as 1000° C., and improvedtransverse rupture strength, and improved Weibull modulus compared tothe prior art.

More particularly, a silicon nitride based ceramic composition isprovided preferably having at least 85 volume percent (v/o) beta siliconnitride phase and an intergranular phase which preferably forms about 1to 5 v/o of the composition. In addition to silicon and nitrogen, theceramic contains on an element basis about 1.3 to 3.5 w/o oxygen, about0.16 to 3.15 w/o yttrium, about 0.12 to 2.7 w/o magnesium. Themagnesium, yttrium and oxygen contents are controlled such that on anoxide basis, the invention contains greater than 0.2 w/o yttria, greaterthan 0.2 w/o magnesia wherein the sum of magnesia and yttria is lessthan 5 w/o. Preferably, there is at least 0.5 w/o of each, yttria andmagnesia. Preferably, yttria is less than 4.0 w/o and magnesia is lessthan 4.5 w/o. The sum of magnesia and yttria is preferably at least 1.5w/o on the low end. On the high end, the sum of magnesia and yttria ispreferably less than 3.5 w/o. A preferred composition contains 0.5 to1.5 w/o magnesia and 0.5 to 1.5 w/o yttria. Oxygen contents of about 1.3to 2.2 w/o and 1.3 to 1.9 w/o are also contemplated. The inventionpreferably has a porosity of less than 0.2, and more preferably lessthan 0.1 v/o.

More preferably, silicon nitride forms at least 95 and most preferablyat least 96 v/o of the composition. Yttrium and magnesium are preferablyadded to the composition as yttria and magnesia.

Ceramic cutting tools for the high speed chip forming machining ofmetallic materials, such as cast irons, are made composed of theforegoing compositions.

These cutting tools in accordance with the present invention have aflank face and a rake face over which chips formed during chip formingmachining flow. At a juncture of the rake face and flank face a cuttingedge is formed for cutting into metallic materials at high speeds toform chips.

These and other aspects of the present invention will become moreapparent upon review of the drawings which are briefly described belowin conjunction with the detailed description of the invention whichfollows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a cutting tool in accordance with thepresent invention.

FIG. 2 shows the hardness of an embodiment of the present invention as afunction of temperature.

FIG. 3 is a scanning electron micrograph of an embodiment of the presentinvention showing its microstructure.

FIG. 4 is a scanning electron micrograph of a fracture surface of anembodiment of the present invention.

FIG. 5 is a graph of cutting tool nose wear as a function of the numberof cutting passes for a prior art tool and two embodiments of tools inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, FIG. 1 shows a preferredembodiment of an indexable ceramic metalcutting insert 10 composed ofthe silicon nitride based ceramic material discovered by the presentinventors. The metalcutting insert 10 is preferably used in the highspeed (>500 surface feet/minute) chip forming machining (e.g. turning,milling, grooving and threading) of metallic materials. This inventionis most preferably used in the high speed machining of cast irons (e.g.,gray and nodular irons), and is particularly useful in roughing andinterrupted cutting of these materials where a combination of hightoughness and high wear resistance is required. The metalcutting inserthas a rake face 30 over which chips, formed during high speed machiningof high temperature alloys and cast irons, flow. Joined to the rakesurface 30 are flank faces 50. At the juncture of the rake face and theflank faces 50 is formed a cutting edge 70 for cutting into the hightemperature alloys and cast irons at high speeds. The cutting edge 70may be in either a sharp, honed, chamfered or chamfered and honedcondition depending on application requirements. The hone may be any ofthe styles or sizes of hones used in the industry. Preferably, thecutting edge 70 has a chamfer (i.e., T-land). The cutting insert mayalso be made in standard shapes and sizes (for example SNGN-434T,SNGN-436T, SPGN-633T, SPGN-634T, inserts may also be made with holestherein as well). The chamfer may typically have a width of 0.003 to0.020 inch and an angle of about 20° to 30°.

The metalcutting insert described above is composed of a silicon nitridecomposition in accordance with the present invention. This compositionhas a microstructure of beta phase silicon nitride grains having anintergranular phase or phases disposed between the silicon nitridegrains. The beta silicon nitride grains preferably form at least 85 v/oof the ceramic and more preferably at least 95 v/o. The beta siliconnitride grains have both an equiaxed and an accicular, or needlelikestructure and preferably have a diameter of less than 1 μm.

The intergranular phase preferably forms about 1 to about 5 v/o of theceramic and is preferably a glass which is a product of the sinteringaids magnesia, yttria, and silicon oxide impurities from the siliconnitride.

The sintering aids used are preferably magnesia and yttria. However, itmay be possible to substitute a high temperature oxide such as those ofhafnium and the lanthanide series of elements for all or part of theyttria. It may also be possible to substitute calcia for all or part ofthe magnesia used herein.

There should be at least 0.2 w/o magnesia and 0.2 w/o yttria in thecomposition of the present invention for sinterability. For cuttinginsert applications, preferably there should be at least 0.5 w/omagnesia and at least 0.5 w/o yttria to assure adequate densification,i.e. a density of at least 3.19 g/cm³ or preferably a porosity levelbelow 0.2 and more preferably below 0.1 v/o. A composition containing1.0 w/o MgO and 0.5 w/o Y₂ O₃ has been found to provide adequatesinterability for cutting insert applications. Therefore, it ispreferred that the sum of magnesia and yttria should be at least 1.5v/o.

As sintering aid content goes up, the hardness of the present invention,both at room temperature and elevated temperatures, goes down. It is,therefore, important that the sum of yttria and magnesia be maintainedbelow 5 w/o. Individually, yttria may be as high as 4.0 w/o and magnesiaas high as 4.5 w/o. For the aforementioned reason, it is preferred thatthe sum of magnesia and yttria be less than 3.5 w/o, and more preferablyless than 3.0 w/o, and most preferably less than or equal to about 2w/o. Compositions in the range of 0.5 to 1.5 w/o magnesia and 0.5 to 1.5w/o yttria have been found to have excellent metalcutting performance inthe high speed rough milling of cast irons.

The compositions in accordance with the present invention preferablyhave a Vickers Hardness Number (VHN, 1 kg load) at room temperaturegreater than 1700 kg/mm² and at 1000° C. of greater than 800 and morepreferably greater than 900 kg/mm². The transverse rupture strength ofthe present invention is greater than 150 and more preferably greaterthan 160 ksi in the 3 point bend test and preferably has a Weibullmodulus of at least 15. Young's modulus of the present invention ispreferably at least 300 GPa and more preferably at least 320 GPa. Thethermal diffusivity (cm² /sec) is preferably at least 0.2, and thethermal conductivity (cal./sec-cm° C.) is preferably at least 0.1.

The significant advantages of the present invention are furtherindicated by the following examples which are intended to be purelyillustrative of the present invention.

Cutting inserts of the SPGN-633T style were manufactured using thefollowing techniques. The starting materials, in the proportions shownin Table I were milled for 24 hours with Si₃ N₄ media to obtain a BETsurface area of about 14 m² /g and a particle size range in which atleast 90% of the powder was less than 1 μm. After milling, the powderwas dried, screened and then pelletized using an organic binder.

                  TABLE I                                                         ______________________________________                                                                         Surface                                                 Particle Size                                                                              Nominal  Area (BET)                                   Material   90%< (μm) Wt. %    m.sup.2 /g                                   ______________________________________                                        Si.sub.3 N.sub.4                                                                         1.4          98       10-12                                        Grade SN-E10                                                                  Y.sub.2 O.sub.3 Grade                                                                    2.5          1        10-16                                        "fine"                                                                        MgO Grade  --           1        40                                           Light USP/FCC                                                                 ______________________________________                                    

Grade SN-E10 Si₃ N₄ powder is available from Ube Industries, Ltd., ofTokyo, Japan. This powder is equiaxed, has a mean particle size of about0.2 μm, and is approximately 100 percent crystalline, with greater than95 percent being alpha silicon nitride and the remainder, if any, isbeta silicon nitride. The composition of grade SN-E10 silicon nitride is(in w/o): N>38.0; O<2.0; C<0.2; Cl<100 ppm; Fe<100 ppm; Ca<50 ppm; Al<50ppm; and the remainder Si.

Fine grade Y₂ O₃ is available from Herman C. Starck, Inc., New York,N.Y. This powder is a high purity powder of at least 99.95% by weight Y₂O₃. The maximum weight percent of metallic impurities is 0.05.

Grade Light USP/FCC, magnesia is available from the Chemical Division ofFisher Scientific, Inc., Fair Lawn, N.J. This powder has the followingcomposition: MgO≧96 w/o, Acid insolubles≦0.1 w/o; arsenic≦3 ppm;calcium≦1.1 w/o; heavy metals≦0.004 w/o; iron≦0.05 w/o; lead≦10 ppm;loss on ignition≦10 w/o.

After pelletizing, the material was then pill pressed to form greeninserts of the desired geometry. The green inserts were then heated inair at 600° F. to drive off the fugitive organic binder. Subsequently,the green inserts were sintered utilizing a suitable Si₃ N₄ basedsetting powder for 1 to 2 hours in one atmosphere of nitrogen at1800°-1850° C. The sintered inserts were then hot isostatically pressedat about 1750° C. in a 20,000 psi nitrogen atmosphere to achieve finaldensification. The resulting inserts were then ground to final sizeusing a 100 or 180 mesh grit size grinding wheel for top and bottomgrinding. In this manner, SPGN-633T inserts having a T or K land of0.008"×20° were made. The characteristic properties of this compositionare shown in Tables II, III and IV below:

                  TABLE II                                                        ______________________________________                                        Properties     Invention  Prior Art*                                          ______________________________________                                        Hardness, Rockwell A                                                          Range          93.0-94.0  92.8-93.2                                           Preferred Range                                                                              93.3-94.0                                                      Microhardness VHN (18.5 kg load), GPa                                         Range          14.5-15.5  14.2-14.9                                           Preferred Range                                                                              14.7-15.4                                                      Hot Hardness VHN (1 kg. load) (Kg/mm.sup.2)                                    20° C. 1772 ± 24                                                                             1675 ± 9                                          200° C.                                                                              1663 ± 15                                                    400° C.                                                                              1475 ± 11                                                    500° C.           1248                                                 600° C.                                                                              1397 ± 19                                                    800° C.                                                                              1268 ± 17                                                   1000° C.                                                                              936 ± 16                                                                              646 ± 5                                          ______________________________________                                         *Prior Art composition contains about 2.2 w/o yttrium (2.8 w/o yttria) an     about 1.4 w/o magnesium (2.3 w/o magnesia) for a total yttria and magnesi     content of about 5.1 w/o.                                                

                  TABLE III                                                       ______________________________________                                                      Invention Prior Art                                             ______________________________________                                        Chemical Analysis                                                             O:              1.8-2.9 w/o 3.24 w/o                                          C:              0.09 w/o    0.55 w/o                                          Mg:             0.6 w/o     1.43 w/o                                          Y:              0.7-0.8 w/o 2.19 w/o                                          Ca:             100 ppm     N.A.                                              Zr:             <0.01 w/o   <0.01 w/o                                         Al:             ≦0.2 0.02 w/o                                          Fe:             0.01 w/o    0.01 w/o                                          Density (g/cm.sup.3)                                                                          3.19-3.20   3.20                                              Thermal Diffusivity                                                                           0.205       0.189                                             (cm.sup.2 /sec.)                                                              Thermal Conductivity                                                                          0.114       0.106                                             (cal/sec-cm.sup.0 C)                                                          Crystalline     100% β-Si.sub.3 N.sub.4                                                              100% β-Si.sub.3 N.sub.4                      Phases present:                                                               (as determined by                                                             X-Ray Diff.)                                                                  ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                                        Invention                                                                              Prior Art                                            ______________________________________                                        Fracture Toughness                                                                              7.1-7.5    7.12 ± .04                                    K.sub.IC (E&C) (18.5 kg load                                                  Palmqvist Method) MPa · m.sup.1/2                                    Transverse Rupture                                                                              184.5 ± 10.4                                                                          124.1 ± 15.0                                  Strength (3 point bend,                                                       400 grit surface ground)                                                      (Ksi)                                                                         Weibull modulus   19.5       8.3                                              Young's modulus, GPa                                                                            300-350    293.5                                            Shear modulus, GPa                                                                              135.8      113.1                                            ______________________________________                                    

FIG. 2 shows the elevated temperature Vickers Hardness Number (1 kgload) in kg/mm² as a function of temperature in degrees centigrade. Ascan be seen at all temperatures from room temperature to 1000° C. thepresent invention-has a higher hardness than a prior art Si₃ N₄composition containing 2.8 w/o yttria and 2.3 w/o magnesia.

FIG. 3 shows 10,000× magnification view of a metallographically preparedsurface of the present invention. The β-Si₃ N₄ grains (gray) have aneedle-like or accicular form or an equiaxed form. The intergranularphase (white) surrounds the β-Si₃ N₄ grains and is estimated to formabout 3 to 4 v/o of the material. The needle-like structure of some ofthe β-Si₃ N₄ grains is further emphasized by FIG. 4 which shows ascanning electron micrograph at 5000× of a fracture surface from abroken transverse rupture specimen. From these electron micrographs, itcan be seen that the average diameter of the β-Si₃ N₄ grains is lessthan about 1 μm.

The SPGN-633T inserts were then tested in the fly cut milling of a graycast iron engine block (including 6 cylinder bores and cooling channelsfor a diesel engine) against a prior art Si₃ N₄ composition. The priorart composition contains about 2.2 w/o yttrium (=2.8 w/o yttria) and 1.4w/o magnesium (=2.3 w/o magnesia), for a total magnesia and yttriacontent of 5.1 w/o. The test conditions were:

Speed: 3000 sfm

Feed: 0.006 IPT

DOC: 0.080 inch

Coolant: Dry

Cutter Style: KDPR 8" 30° lead angle (See Kennametal Milling/87Catalogue p. 26 (1986))

Length of Pass: 33.75"/Width: 8"

The results of this test are plotted in FIG. 5, where it can be seenthat both inserts in accordance with the present invention, A (100 grit)and B (180 grit), outperformed the prior art material, by achieving agreater number of passes before failure. All inserts failed by chipping.As shown in FIG. 5, the rate of nose wear in the present invention wasless than that produced in the prior art. Therefore, as clearlydemonstrated by this test, the present invention surprisingly has bothenhanced chipping resistance and wear resistance over the prior art inthe milling of cast iron under the conditions shown above.

Optionally, the cutting inserts in accordance with the present inventionmay be coated with a refractory coating for improved wear resistance. Itis contemplated that Al₂ O₃, TiC and TiN coatings may be applied aloneor in combination with each other.

Optionally, the wear resistance of the present invention may also beimproved by the substitution of a refractory particulate material for aminor portion of the β-Si₃ N₄ phase in the composition. The refractorymaterial may form from 1-35 v/o, and preferably 1-10 v/o of the ceramiccomposition. Refractory materials which may be dispersed in the βsilicon nitride matrix include the nitrides, carbides and carbonitridesof Ti, Hf and Zr, and tungsten carbide as well, alone or in combinationwith each other.

The present invention is preferably used in the high speed roughing andinterrupted cutting of cast irons. There may also be applications in theroughing and interrupted cutting of superalloys where the presentinvention may perform well. However, most preferably the presentinvention is best utilized in the milling of cast irons under thefollowing conditions:

Speed: 500-4000 sfm

Feed: 0.004-0.020 IPT

DOC: up to 0.25 inch

By way of definition as used in this specification (unless it is clearfrom the context that starting powders are being referred to) and in theclaims appended hereto, the concentration in weight percent of theyttria (Y₂ O₃) and magnesia (MgO) are calculated values based on theconcentration of the metallic elements, Mg and Y in weight percentdetermined by chemical analysis of the densified ceramic. The calculatedweight percent of Y₂ O₃ is equal to the measured weight percent of Ydivided by 0.787. The calculated weight percent of MgO is equal to themeasured weight percent of Mg divided by 0.601. It should be understoodthat no assertion is being made that MgO and Y₂ O₃ exist as separatephases in the densified ceramic. The use of oxide concentrations inconjunction with the final densified ceramic is done merely to provide aconvenient way of distinguishing the claimed invention from the priorart.

All patents and other publications referred to herein are herebyincorporated by reference in their entireties.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A ceramic cutting tool for high speed chipforming machining of a metallic material, said ceramic cutting toolcomprising:a rake face over which chips formed during said chip formingmachining of said metallic material will flow; a flank face; a cuttingedge, for cutting into said metallic material at high speeds to formsaid chips, formed at a junction of said rake face and said flank face;said ceramic consisting essentially of beta silicon nitride phase; andan intergranular phase wherein said ceramic has at least 0.2 w/o yttriaand at least 0.2 w/o magnesia, wherein the sum of yttria and magnesia isless than 3.5 w/o, and wherein said ceramic has 1.3 w/o to 2.2 w/ooxygen and a density of at least 3.19 g/cm³.
 2. The ceramic cutting toolaccording to claim 1 wherein the beta silicon nitride phase forms atleast 85 v/o of said ceramic.
 3. The ceramic cutting tool according toclaim 1 wherein the sum of yttria and magnesia is at least 1.5 w/o. 4.The ceramic cutting tool according to claim 1 having a hardness at roomtemperature greater than 1700 kg/mm² and at 1000° C. the hardness isgreater than 800 kg/mm².
 5. The ceramic cutting tool according to claim1 having a transverse rupture strength greater than 150 Ksi.
 6. Theceramic cutting tool according to claim 1 having a Weibull modulus of atleast
 15. 7. The ceramic cutting tool according to claim 1 having athermal diffusivity of at least 0.2 cm² /s and a thermal conductivity ofat least 0.1 calorie/sec.-cm⁰ C.
 8. The ceramic cutting tool accordingto claim 1 having a Young's Modulus of elasticity of at least 300 GPa.9. The cutting insert according to claim 1 wherein the yttria is 0.5 to1.5 w/o, and the magnesia is 0.5 to 1.5 w/o;wherein the hardness at roomtemperature is at least 1700 kg/mm² and at 1000° C. hardness is at least900 kg/mm² ; wherein the transverse rupture strength is greater than 160Ksi; wherein the Weibull modulus is at least 15; and wherein Young'smodulus is at least 300 GPa.
 10. The ceramic cutting tool according toclaim 1 wherein the sum of yttria and magnesia is less than or equal toabout 2 w/o.
 11. The ceramic cutting tool according to claim 9 whereinthe sum of yttria and magnesia is less than or equal to about 2 w/o. 12.The ceramic cutting tool according to claim 1 wherein the yttria is 0.5to 1.5 w/o, and the magnesia is 0.5 to 1.5 w/o.
 13. The ceramic cuttingtool according to claim 12 wherein said ceramic has 1.3 to 1.9 w/ooxygen.
 14. The ceramic cutting tool according to claim 10 wherein saidceramic has 1.3 to 1.9 w/o oxygen.
 15. A ceramic consisting essentiallyof:Beta silicon nitride phase and intergranular phase, wherein saidceramic has greater than 0.2 w/o yttria, greater than 0.2 w/o magnesia,wherein the sum of yttria and magnesia is less than 3.5 w/o, whereinsaid ceramic has 1.3 w/o to 2.2 w/o oxygen and a density of at least3.19 g/cm³.
 16. The ceramic according to claim 15 wherein the magnesiais between 0.5 to 1.5 w/o, and the yttria is between 0.5 to 1.5 w/o. 17.The ceramic according to claim 15 wherein the sum of yttria and magnesiais less than or equal to about 2 w/o.
 18. The ceramic according to claim17 wherein oxygen is 1.3 to 1.9 w/o of said ceramic.
 19. The ceramicaccording to claim 16 wherein the beta silicon nitride phase forms atleast 85 v/o of said ceramic.
 20. The ceramic cutting tool according toclaim 1 further comprising:a refractory coating on said cutting tool.21. The ceramic cutting tool according to claim 20 wherein saidrefractory coating includes Al₂ O₃.
 22. The ceramic cutting toolaccording to claim 1 wherein said beta silicon nitride phase forms atleast 95 v/o of said ceramic.
 23. The ceramic cutting tool according toclaim 10 further comprising a refractory coating on said cutting tool,and wherein the refractory coating includes Al₂ O₃.
 24. The ceramiccutting tool according to claim 23 wherein said beta silicon nitridephase forms at least 95 v/o of said ceramic.
 25. The ceramic cuttingtool according to claim 21 wherein said refractory coating also includesTiN.
 26. The ceramic cutting tool according to claim 24 wherein saidrefractory coating also includes TiN.
 27. A ceramic cutting tool forhigh speed chip forming machining of metallic materials, said ceramiccutting tool comprising:a rake face over which chips formed during saidchip forming machining of metallic materials will flow; a flank face; acutting edge, for cutting into said metallic materials at high speeds toform said chips, formed at a junction of said rake face and said flankface; said ceramic consisting essentially of beta silicon nitride phase;and an intergranular phase wherein said ceramic has at least 0.2 w/oyttria and at least 0.2 w/o magnesia, wherein the sum of yttria andmagnesia is less than 3.5 w/o, and a density of at least 3.19 g/cm³. 28.The ceramic cutting tool according to claim 27 wherein the beta siliconnitride phase forms at least 85 v/o of said ceramic.
 29. The ceramiccutting tool according to claim 27 wherein the sum of yttria andmagnesia is at least 1.5 w/o.
 30. The ceramic cutting tool according toclaim 27 having a hardness at room temperature greater than 1700 kg/mm²and at 1000° C. the hardness is greater than 800 kg/mm².
 31. The ceramiccutting tool according to claim 27 having a transverse rupture strengthgreater than 150 Ksi.
 32. The ceramic cutting tool according to claim 27having a Weibull modulus of at least
 15. 33. The ceramic cutting toolaccording to claim 27 having a thermal diffusivity of at least 0.2 cm²/s and a thermal conductivity of at least 0.1 calorie/sec.-cm⁰ C. 34.The ceramic cutting tool according to claim 27 having a Young's Modulusof elasticity of at least 300 GPa.
 35. The cutting insert according toclaim 27 wherein the yttria is 0.5 to 1.5 w/o, and the magnesia is 0.5to 1.5 w/o;wherein the hardness at room temperature is at least 1700kg/mm² and at 1000° C. hardness is at least 900 kg/mm² ; wherein thetransverse rupture strength is greater than 160 Ksi; wherein the Weibullmodulus is at least 15; and wherein Young's modulus is at least 300 GPa.36. The ceramic cutting tool according to claim 27 wherein the sum ofyttria and magnesia is less than or equal to about 2 w/o.
 37. Theceramic cutting tool according to claim 27 wherein the sum of yttria andmagnesia is less than or equal to about 2 w/o.
 38. The ceramic cuttingtool according to claim 27 wherein the yttria is 0.5 to 1.5 w/o, and themagnesia is 0.5 to 1.5 w/o.
 39. A ceramic consisting essentially of:Betasilicon nitride phase and intergranular phase, wherein said ceramic hasgreater than 0.2 w/o yttria, greater than 0.2 w/o magnesia, wherein thesum of yttria and magnesia is less than 3.5 w/o and a density of atleast 3.19 g/cm³.
 40. The ceramic according to claim 39 wherein themagnesia is between 0.5 to 1.5 w/o, the yttria is between 0.5 to 1.5w/o.
 41. The ceramic according to claim 39 wherein the sum of yttria andmagnesia is less than or equal to about 2 w/o.
 42. The ceramic accordingto claim 40 wherein the beta silicon nitride phase forms at least 85 v/oof said ceramic.
 43. The ceramic cutting tool according to claim 27further comprising:a refractory coating on said cutting tool.
 44. Theceramic cutting tool according to claim 43 wherein said refractorycoating includes Al₂ O₃.
 45. The ceramic cutting tool according to claim27 wherein said beta silicon nitride phase forms at least 95 v/o of saidceramic.
 46. The ceramic cutting tool according to claim 38 furthercomprising a refractory coating on said cutting tool, and wherein therefractory coating includes Al₂ O₃.
 47. The ceramic cutting toolaccording to claim 46 wherein said beta silicon nitride phase forms atleast 95 v/o of said ceramic.
 48. The ceramic cutting tool according toclaim 44 wherein said refractory coating also includes TiN.
 49. Theceramic cutting tool according to claim 47 wherein said refractorycoating also includes TiN.
 50. The ceramic cutting tool according toclaim 27 wherein said chip forming machining consists essentially ofmilling and said metallic material consists essentially of a cast iron.